Conventionally, in order to impart oil resistance to paper, an approach has been taken to make the critical surface tension of a treated surface smaller than the surface tension of an oily substance. Chemicals with such function are called oil-resistant agents, and oil-resistant paper treated with a fluorine containing oil-resistant agent has been mainly used.
For example, as Patent Document 1 presents a fluorine containing oil-resistant agent as a novel oil-resistant agent, those in which a fluorine containing compound such as acrylate or phosphoric ester of perfluorocarbon is used have been mainly used as oil-resistant agents for oil-resistant paper because they are inexpensive and effective.
In the case of oil-resistant paper using a fluorine containing oil-resistant agent, no coating is needed to be formed on the surface of the oil-resistant paper because the oil-resistant agent itself has excellent oil repellency and strong water repellency, and consequently the resistance to air permeability of the oil-resistant paper can be made low.
Recently, however, it has been revealed that when fried food is wrapped with such oil-resistant paper using an oil-resistant agent of a fluorine containing compound and heated up in an electronic oven or the like at a high temperature of 100° C. or higher, harmful gas (fluoroalcohol gas, hydrogen fluoride, etc.) that can be accumulated in the human body is generated, and thus use of the fluorine containing oil-resistant agent has posed a serious problem. It has also been pointed out that even without heating by an electronic oven or the like, similar gas may be generated when such paper is used for packaging a food material having a temperature of 100° C. or higher.
In addition, fluorine containing organic compounds have extremely poor biodegradability and there is a global concern for pollution due to these substances. As a reflection of such danger to human health and impact on the global environment as described above, the use of fluorine containing compounds has posed serious social problems.
Patent Document 2 proposes an oil-resistant paper container made of an oil-resistant paper prepared without using any fluorine containing compound, but prepared by forming on the surface of the paper a barrier layer containing crosslinked polyvinyl alcohol and/or starch and a water resistant additive as main components and by applying to the barrier layer a silicone resin and an adhesive for heat sealing. However, this oil-resistant paper container does not always have satisfactory oil resistance, and further involves a problem such that the production cost of the container is high because silicone resin is expensive.
Patent Document 3 proposes an oil-resistant paper prepared by applying a coating layer containing nonionic or cationic polyvinyl alcohol and a coating layer containing a fluorine containing oil-resistant agent in this order. However, the coating layer of polyvinyl alcohol used in this case has a role of preventing permeation of the fluorine containing oil-resistant agent into paper, which is quite different from the subject matter of the present invention.
Patent Document 4, Patent Document 5 and Patent Document 6 propose oil-resistant paper using acrylic emulsion as an oil-resistant agent. However, these kinds of oil-resistant paper proposed in these Patent Documents require a thick acrylic resin coating for satisfying desired oil resistance, resulting in an extremely high resistance to air permeability so as to impair the properties as a food packaging material. When a food packaging material has a high resistance to air permeability and food is heated or kept warm while being wrapped with the packaging material, the inside of the package is filled with vapor generated from food, and food is moistened with condensed dew, and texture and taste of the food are remarkably degraded as the case may be. In addition, when the food is reheated in an electronic oven or the like while being wrapped with the packaging material, rapidly generated vapor cannot be discharged to the outside and the package may be broken. Moreover, in order to form a coating having sufficient oil resistance, a large coating amount is needed, and consequently a problem of increased costs of packaging materials is caused.
When food is heated in an electronic oven or the like while being wrapped with a packaging material, the easiness in discharging of the vapor generated therein to the outside may be represented by the vapor permeability as well as the resistance to air permeability. As a method for measuring the vapor permeability, there is a method referred to as “the moisture permeability measurement method for moisture-proof packaging material” specified in JIS Z-0208 (1976), wherein the moisture permeability is defined as “the amount of the vapor passing through a unit area of a film material in a specified time.” However, this measurement method takes very long time, and is not suitable as a method expected to be compatible even with the cases involving such problems at the time of actually being used as food packaging materials that vapor is condensed as dew in the package, and the rapidly generated vapor cannot be discharged to the outside and the package is broken while food is heated in an electronic oven. Accordingly, as an evaluation test of the moisture permeability of such food packaging material as the present invention, it is preferable to examine the dew condensation conditions in a package and the package break conditions as observed by actually placing and heating food or a substitute therefor in the package.
On the other hand, in order to ensure high oil resistance, lamination of film on the surface of paper has been generally practiced. However, when a film is laminated, the resistance to air permeability becomes extremely high, and the resulting food packaging material is defective as described above.
To prevent the resistance to air permeability from becoming extremely high, Patent Document 7 proposes an air-permeable oil-resistant sheet including a substrate such as a sheet of paper having pores and a thermoplastic film having pores similar to those of the substrate and being laminated on at least one side of the substrate. It is also proposed to form a laminate of non-woven fabric and paper. However, there has been a problem that such sheets cannot fully prevent edible oil from bleeding to the outside, and excellent oil resistance has not been achieved.
Patent Document 8 proposes an oil-resistant paper using hydrophobized starch. However, for the purpose of achieving sufficient oil resistance by using only hydrophobized starch, an enormous amount of hydrophobized starch is required to be applied, and this is impractical in terms of the cost. In addition, increase in resistance to air permeability due to the increased coating amount also causes a problem. Further, when oil-resistant paper using hydrophobized starch alone is used as a food packaging material, there has been a problem that the starch is dissolved due to vapor generated from the food being heated and adheres to the surface of the food because the starch is easily soluble in water.
Patent Document 9 proposes oil-resistant paper coated with an oil-resistant agent in which polyvinyl alcohol is used or polyvinyl alcohol and a crosslinking agent are used in combination. A small coating amount of this oil-resistant agent can attain a high oil resistance, and hence the resistance to air permeability is suppressed to be lower as compared to other oil-resistant agents containing no fluorine; however, this oil-resistant agent includes polyvinyl alcohol, and hence the suppression cannot be said to be sufficient. Further, application of this oil-resistant agent with a size press causes a problem of staining of the dryer.
Patent Document 10 proposes oil-resistant paper prepared by making non-sized paper uniformly contain starch, polyvinyl alcohol and an acrylic oil-resistant agent. However, this oil-resistant paper is also insufficient in oil resistance as a food packaging material, and accordingly, in order to ensure sufficient oil resistance, a large amount of coating layer is needed to be formed, resulting in a problem that the resistance to air permeability is increased.
Patent Document 11 proposes oil-resistant paper having on a paper substrate two coating layers, namely, a lower layer that is a coating layer composed of a mixture of an elastomer such as rubber latex or a water-retaining/water-absorbing polymer and a gelatinizable starch and an upper layer that is a coating layer composed of a starch decreased in viscosity or a starch derivative. This oil-resistant paper ensures the oil resistance mainly on the basis of starch and elastomer or a water-retaining/water-absorbing polymer, and hence, in order to ensure sufficient oil resistance, resistance to air permeability is forced to be sacrificed; consequently, no oil-resistant sheet material excellent in oil resistance and low in resistance to air permeability has been able to be obtained. Additionally, in this oil-resistant treated paper, the starch is used for the purpose of forming a film, and the resistance to air permeability is out of the scope of consideration.
As described above, the conventional art has never been able to obtain any oil-resistant paper that can simultaneously satisfy desired oil resistance, resistance to air permeability and productivity so as to be suitable as a food packaging material.
[Patent Document 1]: Japanese Patent Laid Open No. 12-026601
[Patent Document 2]: Japanese Patent Publication No. 6-2373
[Patent Document 3]: Japanese Patent Laid Open No. 8-209590
[Patent Document 4]: Japanese Patent Laid Open No. 9-3795
[Patent Document 5]: Japanese Patent Laid Open No. 9-111693
    [Patent Document 6]: Japanese Patent Laid Open No. 2001-303475[Patent Document 7]: Japanese Patent Laid Open No. 11-021800[Patent Document 8]: Japanese Patent Laid Open No. 2002-69889[Patent Document 9]: Japanese Patent Laid Open No. 2004-68180[Patent Document 10]: Japanese Patent Laid Open No. 2005-29943[Patent Document 11]: Japanese Patent Laid Open No. 2005-29941